Skip to main content

Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

See other formats

The cross-section of this fiord is that of a steep-sided, flat-bottomed glacial trough, with tributary hanging valleys. A typical submerged hanging valley on the northeastern side lies beneath the waters of Mummy Bay on Knight Island. The bottom of Mummy Bay slopes from 126 feet at the head to 522 feet at the mouth, S£ miles distant. The depth then increases to 1260 feet in Knight Island Passage, three-quarters of a mile outside. The slope above this hanging valley lip is 126 feet to the mile, while that outside is at the rate of 984 feet to the mile. Little and Hogan Bays near the southern tip of Knight Island also have submerged hanging valley relationships to the main fiord.
These relationships are of especial importance in connection with the origin of the southeastern portion of Knight Island Passage and outer Icy Bay which Grant and Higgins l have explained as due to graben faulting. We dissent from this view and ascribe the straight, steep-walled portion of Knight Island Passage east of Icy Bay and south of Knight Island entirely to glacial erosion. There is no published evidence of faulting, but the passage has every characteristic of an ice-sculptured fiord. The critical features, however, are the submerged hanging valleys. Such forms could be produced by faulting, it is true, but the different discordances of the lips of the submerged hanging valleys of different sizes cannot be explained by faulting, but are perfectly normal results of differential glacial erosion.
The depth of this southeastern portion of Knight Island Passage is less than might be expected in view of the incoming of the expanded glacier of Icy Bay, but is explained by the loss of ice through the distributary channels on the southwestern side of Knight Island Passage.
Latouche, Elrington, Hoodoo, and Flemming Islands, between Knight Island Passage, Montague Strait and the mainland, are separated from each other and from the mainland by narrow fiords called Latouche, Elrington, and Prince of Wales Passages, and Bainbridge Passage—Port Bainbridge, the four distributary fiords which diverted part of the ice of Knight Island Passage from Montague Strait. Latouche Island is a mountainous island but its coast is hot so indented by minor bays and coves as Knight Island is. This may be due to the difference in the country rock, Knight Island being predominating greenstone with subordinate amounts of conglomerate, graywacke and slate, while Latouche Island is mostly slate and graywacke. It is separated from Elrington and Hoodoo Islands on the west, by a deep straight fiord. Latouche Island has its greatest elevations within ^ to £ of a mile of the southeastern side.
Grant and Higgins 2 have stated that Latouche Island was probably nearly or quite overridden by the ice sheet of Prince William Sound. The intensely glaciated, steep, southeastern side of the island forms a striking contrast with the cirque-carved, northwestern side previously described.
Elrington, Hoodoo, and Flemming Islands are each about twelve miles long. All of them, with the exception of Flamming Island, are narrower than Latouche Island, and more irregular, probably because the constituent rock, the same as in Knight Island, has allowed more dissection by stream and ice erosion. They also contrast in irregularity with the peninsula between Bainbridge Passage and Icy Bay where there ia graywacke and slate, though of a different age from that in Latouche Island. Elring-
i Bull. 443, U. S. Gcol. Survey, 1910, p. 16. > Op. cit., p. 19.